It is common knowledge that each of the electrical poles of a circuit breaker comprises at least two electrodes for connecting to an electrical network and current interrupting means. Each of said current interrupting means comprises at least a pair of contacts suitable for acquiring at least two configurations, i.e. coupled and uncoupled.
The circuit breakers also comprise control means, hereinafter indicated for the sake of brevity by the term control, that establish the mutual coupling and uncoupling of said current interrupting means.
The control comprises propulsion means, such as springs or magnets, that provide the energy needed to couple and uncouple the current interrupting means in the poles, according to the methods required. In addition to the propulsion means, the control can comprise suitable control and drive kinematic chains (particularly shafts and/or sliding members, and/or connecting rods) placed between the propulsion means and the moving contacts of the respective poles.
The installer normally chooses a circuit breaker to suit the particular features of the loads and of the stretch of electrical network for which it is intended, using suitable calculations to formulate a set of performance requirements to be met. That is why manufacturers produce families of devices including various sizes, each of which is suitable for covering a particular range of characteristics.
The most common requirements for a circuit breaker can be summarised, using definitions known to a person skilled in the art, in the form of the so-called nameplate data or “specifications”. The following are normally considered among the requirements for a circuit breaker: rated voltage (Ue), rated impulse withstand voltage (Uimp), rated current (Iu), breaking capacity in various conditions (Icu, Ics, Icw), making capacity (Icm), mechanical life, allowable frequency of operation, electrical endurance in standard conditions, proportional loss of electrical endurance after a short circuit, electrodynamic limiting capacity, insulation between the phases, etc.
The circuit breaker's performance depends on the combination of the characteristics of its constituent parts and particularly on those of the control and electric poles. The control provides the energy for contact opening and closing operations according to previously established methods, while the electrical poles—which include the contacts—are the essential means for creating and interrupting the current.
Much research has been done to improve the characteristics of the controls and electrical poles, both individually and as a whole. As a consequence, there are several varieties of said elements available today, each of which is characterised by specific advantages and disadvantages.
In particular, the manufacturer optimises and exploits the technologies available to produce families and sizes of circuit breakers capable of adequately covering the various performance combinations required for the various types of installation.
It is naturally impossible to have specific circuit breakers tailored to every particular performance combination required. Generally speaking, circuit breakers are chosen that have a slightly better performance than is strictly necessary, taking action to reduce or down-rate them where necessary (using a different calibration of the relays and current sensors, for instance). As it is easy to imagine, this procedure is fine for a modest down-rating, but it would not be cost effective to use appliances that are considerably over-dimensioned for the predicted real needs.
The known types of electrical pole are classifiable in at least two main families, which have become well established, i.e. the poles in free air and the so-called sealed poles, which have to be contained in a specific controlled environment.
The poles in free air are commonly used in moulded-case (MCCB) and air (ACB) circuit breaker devices and are characterised by the presence of the so-called arcing chambers in the vicinity of the contacts. The arcing chambers place the area occupied by the active part of the contacts (where the electric current is created and interrupted) more or less directly in communication with the outside environment. See, for instance, EP0859387. The arcing chambers can comprise a variety of additional elements, described in more detail below. The poles in free air come in versions with single or multiple (e.g. double) current interrupting capabilities. The way in which the contacts move may also vary, being rotatory, translatory or a combination of the two.
The sealed poles are commonly used in high-voltage devices and are normally characterised by the presence of sealed ampoules or chambers surrounding the area of the contacts (where the electrical current is created and interrupted), preventing any free communication between the contacts and the outside environment. Sealed poles are also classifiable in two categories. The first type comprises the so-called vacuum poles, which operate in a severely rarefied atmosphere consisting of known gases; the second type comprises poles in an arc-extinguishing gas, in which case the sealed chamber contains specific gases or gaseous mixtures at a known pressure. Unlike the poles in free air, the sealed poles do not have channels directly communicating with the outside environment, which would be incompatible with their characteristics of air tightness.
It is easy to imagine that the presence or absence of a normal atmosphere in the contact area for the free-air or sealed types of pole gives rise to very different operating conditions.
In particular, the poles in free air must be designed particularly so that they avoid facilitating the formation and so that they instead facilitate the extinction of any electrical and plasma arcs that are well known to be supported by the presence of oxygen and other gases commonly occurring in the normal atmosphere. For this purpose, to ensure the proper operation of the poles in free air, especially when it comes to interrupting high currents, a considerable gap (or extended stroke) must be rapidly created between the active areas of the contacts. Other known optional devices, such as deflectors, foils, filters and gasifying means, can be connected to the arcing chamber to help extinguish the electrical arc, e.g. by diverting the arc towards the areas far from the contacts, absorbing thermal energy, and facilitating the de-ionisation of the plasma and the outflow of gases and filtrates from the circuit breaker, after their residual aggressiveness has been reduced as far as possible.
Given the substantial absence of air or ionisable gases in the area of the contacts, sealed poles operate in very different conditions. In fact, this situation determines a more or less marked immunity to the formation of electrical arcs in the area where the electrical current is interrupted, even when high currents are interrupted are during short circuits, offering the advantage of a perfect operation even with relatively small displacements between the contacts (i.e. a reduced stroke). On the other hand, for sealed poles it is essential to guarantee that the controlled environment (the positive or negative relative pressure tightness) is maintained. Sealed poles also have the advantage of producing virtually no ionised gas emissions or high temperatures in the outside environment, thereby substantially preventing any risk of fire or contamination of the surrounding environment or other parts or accessories of the circuit breaker or other equipment in the vicinity (e.g. the electric switchboard containing the breaker, or other devices installed on the board).
Specifically to support the above-described different electrical and physical principles, which distinguish the operation of circuit breakers with poles in air from that of circuit breakers with sealed poles, and particularly the different needs concerning the relative displacement between the contacts in the closed and open (or tripped) positions, two separate families of controls have also been developed and become well-established, i.e. the so-called controls for poles in free air and the so-called controls for sealed poles. In particular, the controls for poles in free air are of the so-called extended-stroke type, while the controls for use with sealed poles are of the so-called reduced-stroke type.
The most obvious difference between these two types of control consists in the different extent of the stroke that they must impose on the moving contacts in order to complete a circuit breaking operation. Said stroke is normally induced by the combined movement of a main shaft and a suitable intermediate operative connection member (e.g. a connecting rod) between the shaft and the moving contacts.
Another clear difference between the known controls for poles in free air and those for sealed poles concerns the direction of the movement imposed on the moving contacts: it is usually substantially horizontal in circuit breakers with poles in free air and substantially vertical in circuit breakers with sealed poles.
Another natural difference between the two types of control concerns the different dielectric conditions and needs, and the presumable presence or absence of electrical arcs in the vicinity of the poles.
Depending on the type of electrical pole chosen for a given circuit breaker, it becomes necessary to design a corresponding control that is capable of ensuring the circuit breaker's operation, guaranteeing the level needed for each of the declared performance requirements.
In short, the control must be compatible with the constraints and demands relating to the kinematic, dynamic, energetic and dielectric isolation features that, depending on the type of pole chosen, may differ in each case, and may even be in contrast with one another.
The different dielectric demands for poles in free air and sealed poles also entail different choices concerning the materials used; for instance, insulating materials are used to make the arc extinguishing chambers of circuit breakers in free air, while a metal is typically chosen for the ampoules (or sealed chambers) destined for use in circuit breakers with sealed poles.
From the point of view of performance, it has been demonstrated that, in low-voltage circuit breakers, overall size and manufacturing cost being equal, the poles in free air are generally preferable when an excellent short-circuit breaking and current limiting performance is needed, whereas sealed poles are preferred when a particularly prolonged and heavy working life is to be expected, and also for installations at sites with a aggressive atmosphere.
In conclusion, the different needs identified have given rise to consolidated, distinct design and manufacturing solutions for the controls, depending on whether they are destined for use in circuit breakers with poles in free air or with sealed poles.
The poles and the control generally constitute the most important and noble parts of a circuit breakers and must be perfectly compatible with one another. The synergy required between these two elements has led to an industrial approach in which the design and manufacture of circuit breakers with poles in free air or sealed poles are completely separate, specialised processes. This need for separation explains why manufacturers have traditionally foregone the chance to exploit even the marginal compatibility of the less noble and characteristic parts of a circuit breaker (such as the outer case, the accessories and the safety devices) in favour of a complete specificity of all the parts concerned.
In short, if a manufacturer wishes to produce ranges of circuit breakers both with poles in free air and with sealed poles—in order for instance to cover not only a wide range of certain specifications, but also different combinations of these specifications—then, according to the state of the art, the manufacturer is practically obliged to give up any opportunities to standardise component parts of the two families.
In particular, there are no devices available that belong to both types of family, or that offer any appreciable degree of mutual interchangeability between their component parts.
This manufacturing inflexibility is unavoidably translated into the practical need, for the manufacturer, to have separate design resources, technologies and production lines for the two types of circuit breaker and the related accessories, ultimately giving rise to economic costs that cannot fail to have a fallout on the final cost of the devices.
In addition to the economic problem, there is also a practical fallout for users of the two types of device, who are obliged to use separate ranges of accessories and store spare parts for both families of equipment.